Subtract.
There are many ways to solve this problem. Let's see two ways we can solve it. Place Value We can think in terms of hundredths: $\begin{aligned} &\phantom{=}1.64 - 1\\\\ &=1.64 - 1.00\\\\ &=164\text{ hundredths} - 100\text{ hundredths}\\\\ &=64\text{ hundredths}\\\\ &=0.64 \end{aligned}$ Place value table Let's line up the numbers by their decimal places. Ones $.$ Tenths Hundredths ${1}$ $.$ ${6}$ $4$ $1$ $.$ ${0}$ $0$ Now, let's subtract each place value column. Ones $.$ Tenths Hundredths ${1}$ $.$ ${6}$ ${4}$ $-$ $1$ $.$ ${0}$ $0$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $0$ $.$ $6$ $4$ $0.64 = 1.64 - 1$